The invention relates to mobile telephone systems. A subscriber of a mobile telephone system, i.e. a subscriber station, such as a radio phone or a mobile station, may be registered in a radio network or system, whereby it communicates with the radio network via system channels comprising control or traffic channels maintained by the base stations of the radio network.
Apart from system channels of mobile telephone systems, direct mode channels can also be used, i.e. direct mode operation is applied. Mobile stations using direct mode operation do not communicate directly with a radio network or its base stations. Direct mode channels are frequencies at which radio phones or other means of communication are able to communicate without the system either directly with one another or, via repeater stations, with base stations of the system or with other mobile stations.
Direct mode channels are typically used in cases where e.g. a plural number of hand-portable phones communicate with one another at such a distance from the base station that system channels cannot be used.
Direct mode channels are also useful in the addition of capacity when traffic increases rapidly (incident) in one part of the service area of the system, e.g. at one point of the radio network.
A direct mode channel is also called a direct or simplex channel, or a simplex connection. A direct mode channel is a channel that is typically not utilised by the system at all. It may be, for example, a channel with the same channel spacing as the channels of the system, e.g. 12.5 kHz or 25 kHz. Of the radio phones operating on the direct mode channel, the transmitting station has tuned its transmitter to the channel and transmits speech or data information. The other radio phones set to direct mode operation have tuned their receivers to the same channel, whereby they are able to receive the transmission directly.
On a direct mode channel, both analogue modulation and digital modulation can be used. A radio phone transmitting on the channel can also send signalling information, such as information about access rights and priorities or about a group operating on the channel, or data traffic. On a direct mode channel, messages can be encrypted, or speech can be transmitted in clear form.
Subscriber stations operating in direct mode communicate with other subscriber stations on a direct mode channel without being in direct contact with base stations of the radio network. Subscriber stations in the direct mode, however, can communicate with the radio network via repeater stations. A repeater station is a means in which two radio units are interconnected. A repeater station typically comprises two transceivers. A repeater station transmits the information messages transmitted on a direct mode channel to the desired network elements, e.g. base stations of the radio network, which forward the information messages to the exchanges of the radio network. The information messages transmitted by a repeater station may be e.g. speech, or data or signalling messages. When a repeater is used on a direct mode channel, e.g. semiduplex communication is employed.
It should be noted that in direct mode operation, repeaters/repeater stations can also relay traffic between two mobile stations located within the coverage area of the repeater concerned but not necessarily within the coverage area of each other, or function as a gateway between the mobile communication system, especially a base station of the system, and mobile stations communicating on a direct mode channel.
In prior art solutions, e.g. as described in the TETRA (Trans European Trunked Radio System) standard, communication on a direct mode channel is based on group or individual identifiers. Direct mode channel operation is disclosed e.g. in RES 6.6(93)096 Version 0.0.8, Jan. 1995, Technical Requirements Specification, Part 4: Direct Mode, ETSI 31 pages. A repeater station operating on a direct mode channel can also be allocated to a certain group identifier. In such a case, the repeater station relays only traffic marked with this group identifier. Optionally, a repeater station can also be allocated to a plural number of group identifiers.
An open direct mode function, i.e. a direct mode function available to all subscribers, is provided by assigning a single group identifier to all the mobile stations and repeater stations operating on a direct mode channel. In the present invention, however, group identifiers are used for differentiating between organizations/suborganizations/communication groups communicating on a single direct mode channel.
A repeater station is often used such that it is activated to a certain direct mode channel for a group identifier, i.e. for a certain communication group, for mobile stations of that group. If a direct mode channel is reserved for a group identifier that is other than the one with which the base station seeking for activation and the mobile stations having the same group identifier and attempting to operate on a direct mode channel are marked, then the repeater station knows to move to another direct mode channel. After finding a free direct mode channel, the repeater station is activated to this new direct mode channel as a repeater station, and it indicates its group identifier in the identifier transmission.
The problem with the prior art solutions is that the mobile stations activated to the original direct mode channel and listening to that channel do not notice that the repeater station finds a free direct mode channel where it starts to relay traffic between mobile stations, and so the mobile stations cannot follow the repeater station to the direct mode channel where the repeater starts to operate.
This calls for a method of moving the mobile stations remaining on the original direct mode channel to a new direct mode channel reserved for their own group identifier.
More generally, the problem of the prior art is how to pick up the desired mobile stations, e.g. those listening to a certain group identifier, onto a certain direct mode channel from another direct mode channel or other direct mode channels. The use of direct mode channels is very dynamic, so advance planning does not ascertain that the mobile stations are on the same direct mode channel.
In a mobile communication system, mobile stations are allocated to a channel by the system, which means that the system switches the mobile stations to a channel in accordance with their group or individual identifiers. This is not, however, possible in direct mode operation, since here the system has no control over the mobile stations.
In direct mode operation, it is possible for a mobile station to scan a plural number of direct mode channels, looking for a transmission provided with its own group identifier. The basic idea of scanning is that if a transmission provided with a group identifier of a certain group is transmitted on a direct mode channel, and if the mobile station is on that direct mode channel at that particular moment, then the mobile station moves to the direct mode channel concerned.
In prior art, the problem with scanning is that it is difficult to implement in a watertight manner when there are more than two direct mode channels. Namely, it is unlikely that an identifier transmission will be received during the scanning phase, since a mobile station scanning direct mode channels is most of the time switching from one direct mode channel to another or is on another direct mode channel. The time used by a mobile station for scanning or listening to a certain direct mode channel is thereby very short as compared with the length of the transmissions on the direct mode channel. Also, scanning requires that a mobile station should continuously change frequencies, and this consumes power, so it is not a method to be recommended for hand-portable phones. Further, scanning makes it possible for a mobile station to move to another direct mode channel for traffic marked with the same group identifier, but it does not make it possible to pick up all the mobile stations scanning the same group identifier onto one and the same direct mode channel.